There are words every author knows to avoid. Lists abound on the internet, software like prowritingaid.com will tell you how often you use them and even suggest how many you should cut. You will note, it does not say to cut them all. In many ways life would be easier if you could.
This refers to an object in hand. These is the plural version.
This bird I am holding, is better than those birds in the bush.
As you can gather from the sentence, those refers to objects not in hand, it is the plural version of that.
Both this and that are specifiers (Not the technical term, but it works. Technically that opens a restrictive clause.). It is this bird, not the one on my head, or the one hiding in my pocket. It is better than those birds in the bush, not the ones flying overhead, or frozen in the display case at the food market.
To be grammatically correct you use a specifier any time you want to high light a particular object, and that’s where we get into trouble. Just how much specificity do we need in our story?
Gerald struck the troll with the sword that cut through rock.
Here it is important we know the sword is the one that cuts through rock. Not the one that cuts through paper, or the one that cuts through scissors. It is possible to define the sword without using that. ‘the sword forged to cut rock’ would work.
Sometimes we don’t need to be specific for the story.
Glenda handed the troll the cup of tea that she’d just poured.
While it is possible she handed the troll a cup poured last week, I expect it would be show more strongly in the text.
My general rule when editing, either mine or other writers’ work is to cut any ‘that’ or similar word which you can take out without damaging the sentence. For example in the the Glenda sentence above, you could cut that and the sentence would still be perfectly clear.
Following this rule allows us to eliminate a large chunk of superfluous that. We can always rewrite the sentence as we did with the sword to circumvent the need for that.
What is vital is to use that when you need to. When specificity is important and writing around that will take too many words, and when eliminating the that makes the sentence unreadable.
You will likely use that in dialogue more than narrative, since it is a common oral habit. Like the number of contractions, the use of that/this/these/those helps control the level of formality. The more correct grammatically, the more formal.
One pitfall to avoid is using one of these words without clearly connecting it to the thing being specified. This is a common mistake.
If you look at the sentence This is a common mistake. You know what the mistake is as we named it in the sentence before. In fact, it is still redundant, but that’s a discussion for another day. If you wrote. This is a common mistake. without the context of the previous sentence it no longer makes sense. The reader is not told what this specifies. I wrote a paper for university which ended up being unreadable because I used this without connecting it.
Which is not a simple replacement for that. That specifies a particular and the action is restricted to that particular.
The troll broke the tea cup that Glenda gave her.
There may be other tea cups on the table or in the cupboard. They didn’t get broken, only the tea cup given to the troll by Glenda.
The troll broke the tea cup which Glenda gave her.
In this case the tea cup Glenda gave the troll got broken, but the ones on the table or in the cupboard make also have been broken. Which does not restrict the action to a particular object, but places an object into a more general action.
If it isn’t important to either single out one tea cup, or to place the tea cup into a general catastrophe of tea cup breaking, you could leave out the this or that completely. Think of that as a pointing finger that tea cup points to one tea cup and only one. If you want to point to more than one, use those. Which doesn’t care which tea cup, anyone will do.
One last thing. When specifying a person, use who.
The troll who broke the tea cup was sorry.
Who specifies the initiator of the action (subject). Whom specifies the receiver of an action.
The troll to whom Glenda gave the tea cup was sorry.
The quick and easy way to determine between who/whom is to look for the preposition. to whom, for whom, at whom, to whom etc. Who never requires a preposition. This works wherever you place the preposition in the sentence.
So, fast recap. That and its family specify a particular and restrict action to that particular. Which doesn’t restrict the action. Who is for people, and whom is used with prepositions. Anytime you can do without, you’re probably best to cut, but don’t be afraid to keep that word when it is essential to the meaning.